This invention relates to apparatus and method for processing video type signals. More particularly, this invention relates to the processing of signals using digital techniques for the purpose of synchronizing two unrelated video signals on a field by field basis.
In the field of video broadcasting, stringent requirements are placed by the broadcasting authority on both monochromatic, commonly termed black and white, and color signals to be transmitted. In the NTSC color system, for example, the horizontal frequency H = 15, 734 HZ of the video signal must be held to a tolerance of .+-..04Hz while the color frequency f.sub.c = 3.58 .times. 10.sup.6 HZ must be held to a tolerance of .+-. 10 HZ. In addition, for color broadcasting, the color frequency f.sub.c must equal 227.5 H with successive lines having color burst with a phase difference of exactly 180.degree..
For video signals originating in the broadcast studio, the required frequency and phase relationships are maintained to the specified accuracy by using a studio sync generator and locking the entire studio equipment, e.g., TV cameras, video tape recorders, editing equipment and the like to the composite sync, composite blanking and burst signals produced by the studio sync generator, which typically contains a crystal controlled precise clock source. This arrangement suffers from the disadvantage that each piece of studio equipment must be hard wired to the studio sync generator by means of feedback loops in order to insure that any frequency drift in the individual units is corrected before signals emanating from signal sources are output to the transmitter link. In large studios, this requires a complex network of interconnecting cables which introduce signal losses which must be compensated for and which are prone to mechanical wear and breakdown.
It is frequently desirable, and sometimes necessary, to couple a remote source of video signals to the central broadcast studio in order to provide remote TV coverage which may be live or pre-recorded. This is typically accomplished by dedicating a communication link between the remote site and the central broadcast studio over which the video information is transmitted, such as microwave repeater station or telephone lines. Since each remote unit must be synchronized to the studio sync generator, it has been necessary to provide precise timing standards at each remote location and additional dedicated communication links for synchronizing the remote timing sources with the studio sync generator. This synchronizing procedure is time consuming and requires highly skilled personnel and costly timing equipment, e.g. rubidium clock sources, at both ends of the communication link. Moreover, due to the delays introduced by the transmission of the signals along finite distances and other factors, phase and frequency errors are nevertheless introduced into the remote signals which result in signal degradation when switching from one remote source to the central studio source, or from one remote source to another remote source. Efforts to provide a low cost video synchronizing system which can be quickly initialized and which is free of such signal degradation have not met with wide success.